L, K = map(int, input().split())
if L <= K:
    print(0)
else:
    q = L / (K * 2)
    if q == 1:
        q = 0
    elif L % (K * 2) == 0:
        q = (int(q) - 1) * K
    else:
        q = int(q) * K
    print(q)